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<title> Scott Smith's Research Interests </title>

<h1> Scott Smith's Research Interests</h1> 


There has been a large body of important foundational research done in
the area of programming language semantics.  My goal is the practical
application of this work to full-featured programming languages,
languages that have features such as memories, object-orientedness,
rich notions of type, exceptions, and concurrency.  Such a theory
should include as its products a rich theory of equivalence on
programs, as well as logics of programs.

<p>

There are three reasons why this line of research is important.  First
and most obviously, it allows full-featured languages to be
unambiguously specified and remove flaws from their design.  I do not
believe we will ever be able to prove all programs correct, but we
certainly should be able to prove all language designs sound.  Second,
a semantic approach can effectively be used to clean up
inconsistencies in language design, and to provide compiler writers
and programmers with concrete notions of program equivalence and
logical assertions about programs to clarify their activities.  Lastly
(and the topic that interests me most now), novel ideas in language
design and analysis can be effectively pursued in this framework,
because it is possible to carefully develop the ideas for extremely
small languages and scale them up, something that is not possible if a
working compiler must always be an objective.

<p>

Some of the big challenges remaining beyond scaling up existing
results include modelling dynamic systems (those which dynamically
allocate new memory or other objects), open systems (a system where
not all of the computation is occurring locally), and object-oriented
systems.

<p>

<hr>
<h1> Active Research Areas </h1>
<p>

<p><h3>Constrained Types for Object-Oriented Programming Languages</h3>
My main research interest now is in the area of constrained typing.
Constrained types are particularly interesting because

<ul>
<li> They naturally generalize Hindley-Milner type inference to a form
based subtyping instead of type equality.

<li> The types inferred are very expressive: they appropriately type
binary methods in object-oriented programming languages.
</ul>

Our <!WA0><a href="ftp://ftp.cs.jhu.edu/pub/scott/sptio.ps.Z"> OOPSLA '95
paper</a> introduces constrained types with some examples; for the
full technical details, see our <!WA1><a
href="ftp://ftp.cs.jhu.edu/pub/scott/ooinfer.ps.Z"> MFPS paper</a>.

<p>
More details on this work may be found on the <!WA2><a href =
"http://www.cs.jhu.edu/hog/"> Hopkins Objects Group</a> home page.

<p><h3>Distributed Object-Oriented Programming</h3> 

Another project I have been involved with concerns
distributed/concurrent object-oriented computation.  This project has
been performed in collaboration with Gul Agha, Ian Mason, and Carolyn
Talcott.  Here are a few features of what we have accomplished thus
far.

<ul>
<li> A rigorous reworking of the Actor model of computation
<li> Operational semantics of executions given.
<li> Only fair executions considered, for unfair executions never
arise in practice.
<li> Observational equivalence and methods for proving observational
equivalence in the presence of fairness are defined.
<li> Explicit, dynamic modeling of external agents and their
interactions. 
</ul>
<p>
The latter two are, we believe, the key contributions, as little
current research addresses these issues. 
<!WA3><a href = "ftp://ftp.cs.jhu.edu/pub/scott/ffac.ps.Z"> Here</a> is 
a paper on this topic.

<hr>
<h1> Less Active Research Areas </h1>

<p><h3>Semantics for Typed Object-Oriented Programming</h3> Developing
a sound semantics for typed notions of inheritence is a long-standing
research problem.  We believe state is a critical component of OOP,
and we thus directly model objects with state.  Most of the current
research in typing OOP has been for purely functional languages.
Although this gives a good first approximation to the problem, enough
progress has been made in this arena that it is time to work directly
over a language with state.  Our paper in <!WA4><a
href="ftp://ftp.cs.jhu.edu/pub/scott/itoopls.ps.Z"><it>Lisp and
Symbolic Computation</it></a> represents our (final?) statement on
this topic.


<h3>Towards Complete Operational Semantics</h3><p> The aim of this
project is to accomplish a degree of unification between operational
and denotational approaches to programming language semantics by
recasting classic denotational concepts inside a purely operational
framework.  These concepts include notions of ordering <it>a <=
b</it>, directed set, complete partial order, monotonicity,
continuity, least fixed point principle, finite element,
omega-algebraicity, and fixed point induction.  The purpose of this
approach is to give full faithful semantics to languages for which
denotational semantics fails.  To date this approach has been
developed for functional languages, and preliminary extensions to
languages with memories have shown possibilities.  The problem of
finite elements for languages with memories is currently open,
however.  <!WA5><a href = "ftp://ftp.cs.jhu.edu/pub/scott/fosdt.ps.Z">
Here</a> is a paper on this topic.

<p><h3>Asynchronous Digital Circuit Synthesis</h3> 

In collaboration with Amy Zwarico we have defined a formal language
for specifying asynchronous digital circuits that is based on Hoare's
CSP, and verified a translation of these circuits to hardware devices
(collections of gates).  The translation was proven correct by
defining a formal operational notion of equivalence, and incrementally
translating the specification to the circuit in small steps that
preserve equivalence.  Numerous informal arguments of correctness of
similar synthesis methods exist, but this work is the first complete,
rigorous proof of correctness of such a method.  Some other features
of this work include the following.

<ul>
<li> The translation is defined by a set of rewrite rules, broken into five phases.
<li> A new notion of "translational equivalence" is defined to state how 
a translation preserves meaning when the language itself is changing.
<li> Only fair executions considered, for gates are inherently fair.  
This is some of the first work in circuit theory to consider fairness.
</ul>
<!WA6><a href = "ftp://ftp.cs.jhu.edu/pub/scott/ccsdac.ps.Z">  Here</a>
 is a paper on this topic.